As we all know, the peak startup current of a motor does not last long. Although I have test equipment in the way of clamp meters, scope etc none it of seems to come close to getting a start up current reading.

Is my only option "very expensive" kit or is there another method ?

(I am trying to work out what the difference a mechanical friction brake makes in current draw when starting)

you only need a multi-meter to read the start-up current: measure the motor's DC resistance? and measure the voltage applied to the motor. the start-up current is the voltage divided by the DC resistance.

As we all know, the peak startup current of a motor does not last long. Although I have test equipment in the way of clamp meters, scope etc none it of seems to come close to getting a start up current reading.

Is my only option "very expensive" kit or is there another method ?

(I am trying to work out what the difference a mechanical friction brake makes in current draw when starting)

The method used depends on the current levels, but you don't give any data. One general method is to use a shunt and put a scope across the shunt (careful, you may have to isolate the scope from ground or use a differential amplifier). Another method is to use a clamp on current device such as a Bell CG-100A current gun or that wonderful old vacuum tube HP 428B; but these devices have limited bandwidth, so you might not see what you want to see. There are also pulse transformers used for high current applications (hundreds to millions of amps).

And, you're right -- you can spend a bunch of money to buy fancy scope probes for this. Tektronix and others will be happy to sell you some.

For junk around the house, I just take a chunk of 12/3 Romex and solder the three ends together -- voila, a shunt. Measure the resistance with a four wire ohmmeter or hook it up to a constant current power supply and measure the voltage drop across it. Try not to turn the wire into an inductor. I put a couple of bolts though a board and used these to help hook up my redneck shunt. As with all shunts, keep the currents low enough to where self-heating doesn't change the resistance significantly. If it puts out enough light to read by, it's not being used properly.

The shunt is nothing more than a resistor, ideally with very little reactance. You use one frequently to measure the current in a circuit. For example, you measure the voltage across a transistor's base resistor to find the base current.

If you have access to a storage oscilloscope and can also borrow a current transformer [CT] having the right primary-to-secondary turns ratio you can feed the motor supply active lead through the primary loop (window) of the CT, connect a burden resistor on the CT secondary side and measure the burden resistor voltage on the oscilloscope during the motor starting interval.

Say the peak starting current is 10 amps. You'd need a CT with say a 1:100 ratio and a burden resistor of say 10 ohms to give a peak voltage on the burden resistor of 1 volt.

If you have a CT with a higher ratio you can pass the active supply lead through the CT window a few times to obtain a suitable primary to secondary current ratio.

You'd need to trigger the oscilloscope from the burden voltage signal and have the time base adjusted for a short sweep time in the order of 10's to 100's of milliseconds. It'll be hard to catch anything unless you have a storage type oscilloscope - i.e. one capable of storing & displaying a single shot sweep.

Someonesdad.
I think I'm getting there. Supply the motor via the shunt and measure the voltage drop?
I would still need a tasty meter to get the peak reading - I think.

Click to expand...

Yes, the voltage drop across the (non-reactive) shunt tells you the current through the motor. The usual technique is to capture the transient start-up current waveform with a scope, which you say you have (it will need to be capable of storing the waveform or you'll have to know how to trigger it and get a picture of the trace at just the right time). Use a dual channel scope and get a trace of the voltage across the motor windings too -- then you can calculate power and power factor should you want to.

As mentioned above, these measurements need to be done with either differential amplifiers or you need to float the scope with an isolation transformer and know what you're doing to avoid getting shocked. Ask for help if you're not sure.

There are numerous clamp on ammeters that also claim to be able to capture peak values. But though you get a number, you're never sure how "correct" it is. When someone says they want the "peak" current, an oscilloscope or an analog peak detector are the usual ways of getting it.

One thing you should tell us is why you're interested in measuring the starting current. If it's just curiosity, that's fine. But if you're using the information to make some kind of decision, there might be easier ways to get to where you want to go.

The motors have to stop and not creep. When they are new they stop well using their capacitor brake circuit but as they are getting older some have been fitted with a constant friction device on the rotor. I would like to see what sort of effect this device is having on the current draw as it would seem that the contactors that supply the power don't seem to last as long as they used to.

If you have access to a storage oscilloscope and can also borrow a current transformer [CT] having the right primary-to-secondary turns ratio you can feed the motor supply active lead through the primary loop (window) of the CT, connect a burden resistor on the CT secondary side and measure the burden resistor voltage on the oscilloscope during the motor starting interval.

Click to expand...

This sounds like the sort of thing I should be doing but it might be a bit out of my clever level. I will at least look into getting a CT, the right one is another thing.
Cheers for your help

it would seem that the contactors that supply the power don't seem to last as long as they used to.

Click to expand...

This makes sense -- you're concerned that the friction device is adding to the required starting torque, leading to lower life of the contactors because of the increased starting current.

Unless a fast current probe is available, I like t_n_k's suggestion of using a transformer the best, as it's the safest technique. But if it was me, I'd just use a shunt, as I've got a few laying around. If it was on a 120 VAC line like here in the US, I'd just put the shunt on the neutral side and measure the shunt voltage with respect to ground. I'd still do the same on a 220 VAC line, but I'd be careful and use a differential amplifier to measure the shunt voltage. Then, in both cases, I'd capture the start-up current and voltage waveforms. These would be the measurements I'd trust the most.

But I also have the Bell current gun and HP 428B I mentioned above and I'd use those first if I could, as they just involve clipping the probe around the conductor. But 428's bandwidth is 400 Hz and the Bell's is 1 kHz, so one wouldn't see fast stuff using these tools.

2 things ..........
The only thing I ever done with a shunt was strap it across the back of an amp meter to get it to measure the rang required. Is that the sort of thing I need or something completley different ?

When measuring you suggest a diff amp. I'm not sure exactly what or how to use one to be honest. Can I not just stick a scope across it ?

Rs is the shunt resistor and VRs is the voltage across this resistor. You really should read this voltage with a differential amplifier to be safe. If you just hook up the typical scope across the two points, one of the scope's input terminals (usually the shield on the BNC connector) is very likely to be grounded and you'll be grounding the power line -- not a good thing. If you know what you're doing, you can float your scope with an isolation power supply, but then the scope's chassis may be at power line potential. I've done this before I had a differential amplifier, but I was very careful, as I knew what the risks were. I use a Tektronix AM502 differential amplifier or the more modern B&K PR-60 differential amplifier (this probe is rated to 700 V on the 100X range).

It goes without saying that you could connect a small 1:1 transformer to the resistor to provide the isolation if the transformer is rated for the line voltage. Then you can hook the transformer's output to the scope with no worries. But if you need to make such measurements on a regular basis, it's worth investing in the proper equipment.

If you go to wikipedia and look up differential amplifier, what you'll see is correct, but not typically what is meant when one is talking about measurement equipment. What is meant when talking about measurement equipment is something like the two models mentioned above. Numerous old Tektronix vacuum tube scopes and the 5000/7000 series had optional differential amplifier plug-ins. Personally, I wish modern scopes' input amplifiers were all differential and gave you a switch to couple either input to ground.

if you make a large mH air-core choke from fine magnet wire and pass the hot lead of the AC through the center of it you should get a small voltage across the coil. basically a step up transformer. you could make the "sensor" better by wrapping a short length of adequate sized and insulated wire to make the coil, then splice it in with the AC line, etc.